1. Field of the Invention
The present invention relates to an antenna device including a multilayer body, and at least first and second coil conductors provided in the multilayer body, and a radio communication apparatus.
2. Description of the Related Art
In the related art, authentication technologies using radio communication (radio frequency identification (RFID)) are widely used for physical distribution, credit card payments, and so on. In RFID systems, radio communication is performed between an RFID tag or contactless IC card and a reader/writer. This radio communication allows the RFID tag or the like and the reader/writer to exchange internally stored data.
As a short-range radio communication standard, Near Field Communication (NFC) using the 13 MHz frequency band exists. NFC is anticipated to be incorporated into communication terminal apparatuses such as cellular phones. As NFC becomes more commonplace, for example, a user only needs to bring his/her communication terminal apparatus into close proximity to a reader/writer installed in a shop to establish radio communication between the two pieces of equipment, thus enabling easy data transfer or data exchange. For this reason, deployment of NFC to various applications such as contactless credit card payments is currently being considered.
A reader/writer for use in RFID systems such as NFC using the 13.56 MHz band is disclosed in, for example, Japanese Patent No. 4798214.
FIG. 36 is a circuit diagram of a transmitting/receiving circuit to which an antenna device described in Japanese Patent No. 4798214 is applied. This transmitting/receiving circuit includes an IC chip 1, an antenna coil 2, a resonant capacitor C13, a matching capacitor C14, capacitors C15 and C18, a resistor R17, and an inductor L15. The capacitor C15 and the inductor L15 form an EMC filter.
In this transmitting/receiving circuit, when transmitting, a transmitting signal exiting the IC chip 1 passes though the EMC filter and the matching capacitor C14 as indicated by rightward arrows before reaching the antenna coil 2, creating a magnetic field in the antenna coil 2 to thereby perform communication. When receiving, an induction voltage is created by the magnetic field passing through the antenna coil 2, and a receiving signal is transmitted to the IC chip 1 after passing through the resistor R17 and the capacitor C18 of the receiving circuit as indicated by leftward arrows.
As another example of equipment for performing the above-mentioned radio communication (that is, radio communication apparatus), a contactless tag described in Japanese Unexamined Patent Application Publication No. 2001-188890 exists. This contactless tag is capable of recording received data and transmitting recorded data. The contactless tag includes an antenna unit that transmits and receives data, an IC chip that processes data, and an impedance matching circuit for matching impedance between the antenna unit and the IC chip.
In order to minimize radiation of unwanted harmonics from the antenna unit, in some cases, a low pass filter (hereinafter, referred to as LPF) including an inductor element is provided between the IC ship and the antenna unit.
When attempting to build the transmitting/receiving circuit described in Japanese Patent No. 4798214 mentioned above as a module, magnetic-field coupling occurs depending on the arrangement of the antenna coil 2 and the inductor L15, which causes unwanted harmonics to be radiated from the antenna coil 2. When attempting to integrate the antenna unit and the inductor element of the LFP together in the contactless tag described in Japanese Unexamined Patent Application Publication No. 2001-188890, magnetic-field coupling occurs depending on the relative arrangement of these components, causing unwanted harmonics to be radiated from the antenna coil 2.